Cistern



FIG. 1 is a top plan view of a cistern showing our new design;

FIG. 2 is a front view of our new design;

FIG. 3 is a bottom view thereof;

FIG. 4 is a left side view thereof;

FIG. 5 is a right side view thereof;

FIG. 6 is a rear plan view thereof; and,

FIG. 7 is a perspective view of the cistern of our new design.

The broken line portion of the figure drawings is included to showunclaimed subject matter only and forms no part of the claimed design.

We claim the ornamental design for a cistern, as shown and described.